(1) Field of the Invention
The present invention provides a process for the preparation of synthetic mazzite having a small primary particle size.
(2) Description of the Related Art
Mazzite is a kind of natural zeolite, and is structurally characterized in that it consists essentially of gemlinite units and is crosslinked with oxygen 12-membered rings perpendicularly to the C-axis and channels of the 12-membered rings are isolated by walls consisting of alternating ladders of 4-membered and 5-membered rings. The one-dimensional cylindrical channel has 12-membered rings having a minimum free diameter of 7.2 .ANG., 0(6) to 0(6). It is considered that, among zeolites having oxygen 12-membered rings, mazzite has the largest kinetic diameter. Accordingly, mazzite having large pores can be used as an adsorbent, a product stabilized by an aluminum-removing treatment of mazzite, as ultra-stabilized Y-type zeolite, can be used as a cracking catalyst for heavy oils, and a product obtained by an ion-exchange treatment of mazzite can be used as an adsorbent for gas separation. Accordingly, a synthesis of mazzite at a low cost on an industrial scale is desired.
The main uses of zeolites are a dehydrating agent, an adsorbent for gas separation, and a catalyst. Where a zeolite is used as a catalyst, not only the selectivity of a target product but also the life of the catalyst is important. Of course, the longer the life of the catalyst, the longer the application time before regeneration or refilling of the catalyst, and a catalyst having a long life is preferred from the practical viewpoint. It is considered that, as primary particles of a zeolite are small, little coking occurs and the catalyst life is generally prolonged. Accordingly, if a zeolite is used as a catalyst, the primary particle size must be small.
Synthetic zeolites having a structure similar to that of mazzite, that is, synthetic mazzites, which are available at the present, are classified into two types, i.e., ZSM-4 and zeolite .OMEGA.. A tetramethylammonium (TMA) compound is used as the template for the synthesis of each of these synthetic zeolites. The process for the preparation of ZSM-4 is disclosed in Japanese Examined Patent Publication No. 46-27,377. In the examples of this patent publication, the smallest primary particle size is 1 to 2 .mu.m. The process for the preparation of zeolite .OMEGA. is disclosed in the specification of British Patent No. 1,178,186. In the examples, the primary particle size is not disclosed, but the tetramethylammonium compound is used in a large amount.
Namely, a process is not been known for preparing mazzite having a small primary particle size by using a reduced amount of a tetramethylammonium compound.
The silicon, aluminum, and alkali sources are easily available at low cost from sodium silicate or white carbon, sodium aluminate or aluminum sulfate and sodium hydroxide, respectively. On the other hand, a halide and hydroxide of tetramethylammonium are expensive compared with other compounds. Accordingly, a reduction of the amount used of the tetramethylammonium compound will make it possible to prepare synthetic mazzite economically advantageously, and thus we considered that a clarification of the role of tetramethylammonium in the crystallization of synthetic mazzite would result in a reduction of the amount used of tetramethylammonium.
In mazzite, two gmelinite cages are present per unit cell. Synthetic mazzite cannot be synthesized without using a tetramethylammonium compound, and it is considered that one molecule of tetramethylammonium used for the synthesis is present in the gmelinite cage. Therefore, it is deemed that if all of the tetramethylammonium used for the synthesis is included in gmelinite cages of the crystal, the amount used of the tetramethylammonium compound will be reduced. The present invention is based on this assumption.